1. Field of the Invention
This invention relates to a heat roller type fixing apparatus for electrophotographic copying machines, and more particularly to a heat roller type fixing apparatus in which the surface temperature of the heat roller is controlled in accordance with the copying operation.
2. Description of the Prior Art
The heat roller type fixing apparatus consists of a hollow heat roller (referred to as heat roller, merely hereinafter) containing a heating lamp such as a halogen lamp or infrared ray lamp in a hollow space, and a press roller which presses against and rotates with the heat roller, whereby a transfer paper with a toner image which has undergone a transfer process is fed between these rollers to melt and fix the toner image by heat. This type of fixing device is superior as compared with other type in that even when the paper becomes cloggy no fires will occur. It has, however, the disadvantage that the warming-up time (a time it takes for the roller surface to reach a temperature at which the fixing can be done, after the fixing apparatus has been switched on) is long. The warming-up time can be shortened by increasing the capacity of the heating lamp or reducing the heat capacity of the heat roller. In this case, an overshoot (a phenomenon in which the surface temperature of the heat roller reaches its peak beyond the setting temperature) becomes greater. On the other hand, too low a surface temperature of the heat roller will result in an underfixing (a phenomenon in which the toner image does not adhere to a substrate such as plain paper sufficiently and easily comes off it) and too high a surface temperature will produce an offset phenomenon (in which a part of the toner image on the substrate adheres to the heat roller and then sticks to other part of the substrate after one rotation). Thus, accurate control of the surface temperature of the heat roller is very important.
In the ordinary heat roller type fixing apparatus, which have bearings at the ends of the heat roller and also a gear or sprocket at one end of the roller that meshes with the roller driving mechanism, a large amount of heat is radiated through these components resulting in a temperature distribution on the surface of the heat roller such that the longitudinal end portions of the roller surface are lower in temperature than the central portion. Such uneven temperature distribution in the axial direction will cause defective fixing such as underfixing and offset phenomena. To solve this problem a proposal has been made of a heating lamp which has a heat intensity characteristic such that the heat generated at its end portions is greater than that at the central portion. This kind of heating lamp produces a temperature distribution on the heat roller surface which has the highest temperature area between the roller ends and the central portion, due to a great heat radiation effect at both ends, and the low temperature area at the central portion of the roller surface.
In the fixing apparatus utilizing a heat roller with such a surface temperature distribution, it is a known practice to set temperature sensing elements close to the surface of the roller to control the roller surface temperature. In controlling the roller surface temperature, selection of the axial position on the roller surface at which the temperature sensing element is set constitutes an important problem.
It has been proposed to situate the temperature sensing element at the central portion of the roller surface in the conventional fixing devices. In this method, however, when the temperature of the central portion of the roller surface reaches the setting temperature, the temperatures at the areas between the central portion and the ends have far exceeded the setting temperature, causing the so-called overshoot phenomenon. To prevent the overshoot in this method, the heating lamp must be turned off at the temperature lower than the setting temperature and, in the steady state condition, the conduction of the heating lamp must be controlled at the setting temperature. This means that duplicate control circuit must be provided. This tendency becomes greater with increased capacity of the heating lamp.
In addition to these drawbacks the conventional temperature contol method also has the following disadvantage: since the temperature of the roller surface is maintained at one and the same value regardless of whether the device is copying or standing by, unnecessarily large amount of heat is generated during the stand-by, producing adverse effects upon the components of the roller such as deterioration of the quality of lubricating oil in the bearings and of the resin covering the surface of the roller. This is one of the factors limiting the life of the fixing apparatus.